Add consume API

[sagebind]

Add ReadResponseExt::consume and AsyncReadResponseExt::consume methods for reading and discarding a response body as described in #257.

Also update future types used by AsyncReadResponseExt to be Send whenever T is Send as appropriate. Consolidate the newtype futures with a macro that handles conditional Send properly to reduce boilerplate and chance of mistakes.

Fixes #257.

[codecov]

Codecov Report

Merging #284 (1efdc56) into master (dacdad2) will increase coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
+ Coverage   72.87%   73.35%   +0.47%     
==========================================
  Files          51       52       +1     
  Lines        2728     2762      +34     
==========================================
+ Hits         1988     2026      +38     
+ Misses        740      736       -4     

Impacted Files	Coverage Δ
src/response.rs	77.77% <100.00%> (+7.77%)	⬆️
tests/response_body.rs	100.00% <100.00%> (ø)
src/macros.rs	100.00% <0.00%> (ø)
src/client.rs	66.99% <0.00%> (+0.77%)	⬆️
src/body/mod.rs	83.17% <0.00%> (+1.86%)	⬆️
src/lib.rs	76.19% <0.00%> (+2.50%)	⬆️

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